Rolmops is an acidic herring fillet that has been rolled up. The herring fillet is equipped with onions and pickle. The whole is rolled up and secured with a skewer and inserted into a melange of herbs. The herring forms the basis for the manufacture of the roller mops and is a derivative of the acidic herring. Roll mops are made from herring caught after June. After the herring has shot hom or calf, these are too skinny to be sold as Hollandse Nieuwe. Herring is a versatile fish and is widely eaten. The herring is a silver-colored fish and has a dark blue / green back. Depending on the season, the fresh herring is usually frozen on board to manufacture the salty herring or any fresh supply should be inlaid with vinegar.